The present invention relates to switches for controlling currents flowing in both directions such as alternating currents and, particularly, to a solid-state switch formed of semiconductor devices, such as gate turn-off thyristors, connected in series in order that the switch can be operated at a high voltage.
When a solid-state switch is used as, for example, a circuit breaker or, generally speaking, a switch gear in a power system, the solid-state switch must withstand a high voltage because the nominal voltage of the transmission lines is, for example, 3.3 kV through 500 kV, and because several times the system voltage is applied to the circuit breaker after current is cut off. However, a single semiconductor device cannot satisfactorily withstand this high voltage, and thus several semiconductor devices are required to be connected in series to increase the breakdown voltage of the switch. In addition, in the power system, an AC current must be flowing and be cut off.
When gate turn-off (GTO) thyristors are used to make a solid-state switch, diodes for protecting the GTO thyristors from the reverse voltage are necessary to be connected in series with the thyristors because the reverse breakdown voltage of the GTO thyristors is often much lower than the forward breakdown voltage. Some solid-state switches formed of these gate turn-off thyristors have been proposed for controlling the flow of alternating currents. In one of those switches, the gate turn-off thyristors of an arm for forward current and those of the other arm for reverse current are connected in an inverse-parallel (reverse-parallel) configuration. In another one of those switches, diodes are connected in a bridge configuration, and a gate turn-off thyristor is connected at the center of the bridge. Moreover, these solid-state switches as the basic circuit arrangements are connected in series in order to withstand a high voltage that will be applied when the solid-state switches are turned off.